Anisotropic icephobic coating

ABSTRACT

Articles including durable and icephobic polymeric coatings are disclosed. The polymeric coatings include a bonding layer which may contain a substantially fully cured polymeric resin providing excellent adhesion to metallic or polymer substrates. The polymeric coating further includes an outer surface layer which is smooth, hydrophobic and icephobic and, in addition to a substantially fully cured resin, contains silicone comprising additives near the exposed outer surface. The anisotropic polymeric coatings are particularly suited for strong and lightweight parts required in aerospace, automotive and sporting goods applications. A process for making the articles is disclosed as well.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation of U.S. application Ser. No.16/515,093 filed Jul. 18, 2019, now U.S. Pat. No. 11,319,450, thedisclosure of which is incorporated herein in its entirety.

FIELD OF THE INVENTION

The invention relates generally to an article of manufacture comprisinga durable exposed surface which is hydrophobic and has low ice adhesion.The icephobic coating is applied to at least part of an outer surface ofthe article. The inventive coating comprises a layered or gradedstructure containing (i) a bonding layer in intimate contact with ametallic, polymeric or composite substrate and (ii) an outer, exposedicephobic layer containing a silicone additive. The anisotropic anddurable hydrophobic and icephobic coating may contain distinct sublayersand/or a gradual transition from one chemical composition to another.The invention further relates to a process for fabricating the article.

BACKGROUND OF THE INVENTION

Self-cleaning, superhydrophobic and icephobic coatings are of greatinterest for use in transportation, consumer, sporting goods and othercommercial applications where an article is exposed to environmentalelements. Ice accumulation on power lines as well as a variety ofvehicles, such as surface or marine vehicles as well as airplanes, canposes significant challenges. The buildup of ice on aerospace componentssuch as wings, propellers and jet engine parts is a significant safetyconcern and a variety of approaches have been attempted to provide meansof preventing ice accumulation.

Various patent filings address the modification of outer surfaces ofarticles to increase water repellency:

Victor et. al. in U.S. Pat. No. 8,486,319 (2013), assigned to the sameassignee as the present application, disclose super-hydrophobic andself-cleaning articles with a polymeric outer roughened/textured surfacecreated by imprinting exposed surfaces with suitable fine-grained and/oramorphous metallic embossing dies to transfer a dual surface structure,including ultra-fine features less than or equal to 100 nm embedded inand overlaying a surface topography with macro-surface structuresgreater than or equal to 1 μm.

Similarly, various patent filings address coatings applied to outerexposed surfaces of articles, e.g., to render them icephobic:

Putnam et. al. in US 2006/0281861 disclose liquid and/or solidanti-icing fillers and/or oils which are combined with erosion resistantsilicone and/or fluorocarbon elastomeric materials to formerosion-resistant and icephobic coatings. These coatings may be utilizedto prevent ice build-up on various gas turbine engine components,aircraft components, watercrafts, power lines, and telecommunicationlines. Putnam provides no information on adhesion strength between theicephobic coating and the underlying substrate.

Hoover et. al. in U.S. Patent Publication 2007/0254170 disclose aprocess for protecting an article such as a gas turbine engine fan bladeusing an anti-icing coating comprising at least one “polysiloxane freeof additives” and curing the anti-icing composition to form ananti-icing coating exhibiting an ice shear strength of about 19 kPA toabout 50 kPa. The anti-icing coating composition described wasidentified as the commercially available NuSil™ R-2180 product fromNuSil Technology, LLC, Santa Barbara, Calif., USA which is a two-partsilicone elastomer dispersed in xylene. Hoover provides no informationon adhesion strength between the icephobic coating and the underlyingsubstrate.

Byrd et. al. in U.S. Pat. No. 7,202,321 (2007) disclose a method forapplying a polysiloxane-containing coating to a substrate, said coatingpreferably comprising a polysiloxane (amide-ureide). The coating isdurable, long lasting, corrosion-resistant and icephobic. Byrd providesno information on adhesion strength between the icephobic coating andthe underlying substrate.

Butts et. al. in U.S. Patent Publication 2011/0143148 disclose anarticle comprising a weather resistant coating on its outer surfaceexposed to precipitation or airborne debris. The coating comprises twocomponents: (a) a one-part or two-part room temperature vulcanizablepolyorganosiloxane composition; and (b) an ice release-enhancingproportion of at least one polyorganosiloxane composition comprising oneor more silanol or alkoxy-silyl groups and comprising from about 10weight percent to about 85 weight percent of at least onehydroxy-terminated or alkoxy-terminated polyoxyalkylenealkyl radical. Inanother embodiment, an article comprises a weatherable surface exposedto precipitation or airborne debris; and a weather resistant coatingdisposed on the weatherable surface, wherein the coating includes aone-part or two-part addition curable polyorganosiloxane compositioncomprising a resin polymer and a crosslinker, wherein the resin polymerand/or crosslinker comprises an ice release-enhancing proportion ofcovalently bound hydrophilic functionality that contains between about0.5 weight percent to about 40 weight percent of the coatingcomposition. Butts provides no information on adhesion strength betweenthe icephobic coating and the underlying substrate.

Nowak et. al. in U.S. Patent Publication 2014/0162022 disclosestructural coatings with dewetting and anti-icing properties which areimpact-resistant, and coating precursors for fabricating same. Dewettingand anti-icing performance is simultaneously achieved in a structuralcoating comprising multiple layers, wherein each layer includes (a) acontinuous matrix; (b) discrete templates dispersed that promote surfaceroughness to inhibit wetting of water; and (c) nanoparticles thatinhibit heterogeneous nucleation of water. These structural coatings canbe applied by spraying and the use of multiple layers extends thelifetime, as in case the surface is damaged during use, freshly exposedsurface will expose a coating identical to that which was lost. Nowakprovides no information on adhesion strength between the icephobiccoating and the underlying substrate.

Harmer et. al. in WO 2015/094917 disclose substrates coated with amultilayer film having low ice adhesion. The multilayer film comprises afirst layer that is a polymer having a thickness of greater than 0.9 mmand less than 10 mm, a Shore A hardness of less than about 100 and aflex modulus in the range of from 1 kPa to 100 GPa. The second filmlayer of the multilayer film is a polymer that has a thickness of lessthan 100 μm, a water contact angle of greater than 60°. The first secondfilm layers can flex without separating or without forming cracks.Harmer provides no information on adhesion strength between theicephobic coating and the underlying substrate.

Tuteja et. al. in WO 2016/176350 disclose durable icephobic coatings foraircrafts, powerlines, vehicles, marine structures, communicationstowers, outdoor equipment, and the like. The icephobic material maycomprise an elastomeric polymer with a low crosslink density (e.g.,≤1,300 mol/m³) and low initial ice adhesion strength (e.g., τ_(ice)≤100kPa prior to exposure to icing conditions). Furthermore, the icephobicmaterial maintains Tice after 10 icing/deicing cycles that is ≥50% ofthe initial τ_(ice). Introducing optional miscible liquids enhancesinterfacial slippage of chains in the elastomeric polymer. The lowτ_(ice) levels minimize ice buildup and any accumulated ice spalls offduring normal operation. Other icephobic materials include linearpolymers with plasticizers distributed therein or PDMS-silane coatings,both of which are free of any layers of surface liquids. Tuteja providesno information on adhesion strength between the icephobic coating andthe underlying substrate.

Thus, there is a particular need for articles containing an adherent,durable, hydrophobic and icephobic coating on their outer surface.

SUMMARY OF THE INVENTION

A variety of articles are made of metallic materials that are used inatmospheric conditions where their outer surface is exposed to rain,snow and/or ice. It is also well known that relatively thin,grain-refined and/or amorphous metallic coating can be applied to softermetals, including, but not limited to, Al and Ti, as well as polymericmaterials, including, but not limited to, thermoplastics and thermosetswhich may be filled with, e.g., carbon or glass fibers for use inapplications requiring high specific strength and durability. It is alsowell known that articles made from conventional, unfilled/unreinforcedas well as fiber reinforced polymers (FRP) and carbon fiber reinforcedpolymers (CFRP) are frequently employed in outdoor applicationsincluding, but not limited to, sporting goods as well as intransportation systems such vehicles traveling on land, in water or air.

As noted above, it is known to minimize the adverse effect ofatmospheric conditions on articles made of various metals as well asarticles comprising non-metallic substrates, e.g., polymer and polymercomposite substrates, by applying a coating on their outer surface torender them water repellent and/or to lower ice adhesion. For instance,per-fluoroalkyl silanes have been employed in surface coatings to reduceice adhesion. However, compositions of this type lose theirwater-repellency rather quickly and need to be regenerated frequently,i.e., they are considered to contain “consumable” components. Quaternaryammonium siloxane-based materials have been incorporated, e.g., intowindshield washing fluids, but this approach fails to provide a durableanti-icing coating and readily washes away during use. Attempts toutilize silicone rubbers, silicone oils and polyisoprenes have not beensuccessful as they undergo rapid wear, are lost from the outer surfaceand furthermore they take up oils and dirt. Furthermore, typicaladditives used to lower ice adhesion including, but not limited to,silicones, also reduce the adhesion of the coating to its underlyingsubstrate causing the coating to delaminate and spall off during use.

The inventors of the present disclosure have recognized that selectedproperties desired of the icephobic coating on the exposed outer surfaceare diametrically opposed to selected properties required on the innersurface of the icephobic coating which is in intimate contact with theunderlying article/substrate. The inventors have surprisingly discoveredthat a multilayered or graded organic coating applied to the metallic orpolymeric substrates provides unprecedented hydrophobicity,icephobicity, durability and adhesion while providing excellent cohesivestrength. The inventors have furthermore discovered that icephobiccoatings can be designed to be hydrophilic or hydrophobic although, inthis specification, hydrophobic surfaces are preferred as such coatingsrepel water as well as readily shed snow and ice.

The inventors of the present disclosure have also discovered that theouter surface of the inventive coating be preferably smooth (R_(a)<1μm), as textured/roughened surfaces such as textured superhydrophobicsurfaces, under certain conditions, can have increased ice adhesion,particularly in the case of frost formation and only show lowice-adhesion if a liquid, e.g., supercooled water droplets, hit thesurface. In contrast, the inner surface of the inventive coating at anysolid interface is preferably rough (R_(a)>1 μm) to enhance the adhesionbetween the coating and, e.g., the underlying substrate.

It is therefore an objective of the present invention to utilizearticles made of durable polymeric materials or metallic materialscomprising an amorphous and/or crystalline microstructure and renderingthe outer surface hydrophobic and/or icephobic by applying an adherent,polymeric, durable, hydrophobic and/or icephobic coating.

It is an objective of the present invention to provide articles whereinthe icephobic material coating surface extends over between 1% and 100%of the total exposed outer surface of the article.

It is an objective of the present invention to provide icephobiccoatings having a total thickness of at least 10 microns, preferably inthe range of 25 to 250 microns (cured loading: ˜3-30 mg/cm²), preferablybetween 50 and 100 microns (cured loading: ˜6-12 mg/cm²).

It is an objective of the present invention to provide durable, scratchand abrasion resistant, strong, lightweight articles for use in variousapplications including, but not limited to, transportation applications(including automotive, aerospace, ships and other vessels navigating onland, in air, space and on water, and their components), defenseapplications, industrial components, building materials, consumerproducts, electronic equipment or appliances and their components,sporting goods as well as any other outdoor equipment.

It is an objective of the present invention to render the outer surfaceof articles hydrophobic (contact angle for water greater than 90°),preferably greater than 120°, preferably super-hydrophobic (contactangle for water greater than 150°).

It is an objective of the invention to render the outer surface ofarticles self-cleaning by suitably creating a low roll-off angle (tiltangle for water less than 25°), preferably a tilt angle for water lessthan 20°, more preferably less than 10° or even 5°, by an economic,convenient and reproducible process.

It is an objective of the present invention to achieve excellentadhesion between the organic coating and the outer surface of articlesby creating a rough/textured surface at the interface between thearticle/substrate and the applied coating, i.e., generating a surfaceroughness (R_(a)) of more than 0.5 microns, preferably more than 0.75microns, and more preferably more than 1 microns.

It is an objective of the present invention to render the outer surfaceof articles hydrophobic and icephobic by applying a coating with asmooth exposed surface characterized by a surface roughness (R_(a)) lessthan or equal to 1 micron, preferably <0.75 microns, and more preferably<0.5 microns.

It is another objective of the present invention to provide a coatinghaving a polymeric outer layer which at room temperature, in its curedform, does not contain any fluids, e.g., silicone oils.

It is an objective of the present invention to provide lightweightarticles comprising, at least in part, liquid repellent and/orself-cleaning outer surfaces which also display low ice-adhesionstrength with increased wear, erosion and abrasion resistance,durability, strength, stiffness, thermal conductivity and thermalcycling capability.

It is an objective of the present invention to provide an outer surfacewith a low ice-adhesion strength which is largely maintained afterrepeated icing/deicing cycles using, after curing, solid and “permanent”icephobic additions as opposed to, after curing, sacrificial, consumableicephobic additions such as lubricating liquids such as oils whichrapidly get consumed and/or lost from the surface resulting in a rapidrise of the ice-adhesion strength with repeated icing/deicing cycles ortime exposed to the environmental elements due to, among other, wind,rain, and ice erosion.

It is an objective of the present invention to provide coated articleswith an outer surface having an ice-adhesion strength on the firstcycle, as well as on the 5^(th) or 7^(th) cycle, of no greater than 500kPa, preferably no greater than 400 kPa, preferably no greater than 350kPa, preferably no greater than 200 kPa, and more preferably no greaterthan 150 kPa, when measured according to ERDC/CRREL Technical Note 03-4.

It is an objective of the present invention for the outer surface of thecoated articles to have a Shore D-scale hardness of at least 10,preferable 15, preferable 20, preferable 25, more preferably 50, andmore preferably 60.

It is an objective of the present invention for the outer surface of thecoated articles disclosed to have a sand erosion value according tostandard ASTM G76 at 90 degrees of less than 10 mm³/kg, preferably lessthan 7.5 mm³/kg, and preferably less than 5 mm³/kg.

It is an objective of this invention to provide coated articles composedof icephobic, polymeric outer coatings on metallic materials including,but not limited to, Al, Co, Cu, Fe, Ni, Sn, Ti and Zn and their alloys,and/or polymeric materials including, but not limited to, polyamides andcarbon fiber composites, having a “pull-off strength” between thesubstrate/article and the icephobic outer layer according to standardASTM 4541D of at least 250 psi (1.73 MPa), preferably at least 300 psi,preferably at least 400 psi, preferably at least 800 psi, preferably atleast 900 psi, preferably at least 1,000 psi and preferably at least1,200 psi.

It is an objective of this invention to provide coated articles composedof a icephobic polymeric outer coatings on metallic and/or polymericmaterials showing no failure such as delamination according to ASTMB553-71 for service condition 1 (60° C. to −30° C.), preferably servicecondition 2, preferably service condition 3 and even more preferably forservice condition 4.

It is an objective of this invention to provide coated articles capableof withstanding 1, preferably 5, more preferably 10, more preferably 20and even more preferably 30 temperature cycles without failure accordingto ANSI/ASTM specification B604-75 section 5.4 (Standard RecommendedPractice for Thermal Cycling Test for Evaluation of ElectroplatedPlastics ASTM B553-71) for service condition 1 (60° C. to −30° C.),preferably service condition 2, preferably service condition 3 and evenmore preferably for service condition 4.

It is an objective of the present invention to provide coated articleswith at least in part icephobic and liquid repellent and/orself-cleaning outer surfaces for a variety of applications including,but not limited to:

-   -   aerospace parts and components including, but not limited to,        wings, wing parts including flaps and access covers, structural        spars and ribs, propellers, rotors and rotor blades, stators and        stator vanes, rudders, covers, fuselage parts, nose cones, and        landing gear;    -   automotive components including, but not limited to, heat        shields, oil, transmission and brake parts, fluid tanks and        exposed housings including oil and transmission pans, spoilers,        grill-guards and running boards, vehicle chassis parts including        hood, doors and side panels, gas tanks and engine covers;    -   sporting goods including, but not limited to, hockey sticks,        skate blades, helmets, golf shafts, heads, balls and faceplates,        ski and snowboard components including bindings, and bicycle        parts; and    -   industrial/consumer products and parts including, but not        limited to, solar panels, turbines and windmills;

According to exemplary embodiments of the present invention, a method isprovided for manufacturing an article having an exposed surface,comprising at least portions that are rendered hydrophobic and/oricephobic.

Accordingly, in one embodiment, the present invention provides adurable, icephobic, non-isotropic article comprising:

-   -   (i) at least one metallic material layer having a total        thickness of at least 25 microns comprising at least one metal        chosen from the group consisting of Al, Co, Cu, Fe, Ni, Sn, Ti        and Zn;    -   (ii) a layered and/or graded anisotropic polymeric coating        having a total thickness of at least 10 microns applied to at        least part of the outer surface of the metallic material layer        and in intimate, direct contact therewith comprising:        -   (a) a cured first chemical composition comprising a            polymeric resin at an interface between said polymeric            coating and said metallic material layer extending at least            2.5 microns in height from the outer surface of the metallic            material layer;        -   (b) a cured second chemical composition comprising the            polymeric resin and an icephobic material addition forming            an exposed outer surface of said non-isotropic article            extending at least 2.5 microns in depth from the exposed            outer surface which, after curing:            -   (i.b) contains an icephobic material addition composed                entirely of solids representing up to 25% by weight of                the cured second chemical composition;            -   (ii.b) has a Shore D-Scale Hardness of at least 20,            -   (iii.b) has a sand erosion value according to standard                ASTM G76 at an impingement angle of 90 degrees of less                than 10 mm³/kg;            -   (iv.b) is hydrophobic; and            -   (v.b) has an ice adhesion of less than 200 kPa as                prepared and of less than 350 kPa after 5 icing/deicing                cycles when measured according to ERDC/CRREL Technical                Note 03-4;    -   said non-isotropic article exhibiting:        -   (i) no failure after being exposed to at least one            temperature cycle according to ASTM B553-71 service            condition 1; and        -   (ii) the pull-off strength between the metallic material            layer and an exposed outer surface of the polymeric coating,            according to standard ASTM 4541D is at least 300 psi.

Accordingly, in another embodiment, the present invention provides adurable, icephobic, non-isotropic article comprising:

-   -   (i) at least one polymer or polymer composite material layer        having a total thickness of at least 25 microns;    -   (ii) a layered and/or graded polymeric anisotropic coating        having a total thickness of at least 10 microns applied to at        least part of an outer surface of the polymer or polymer        composite material layer and in intimate, direct contact        therewith comprising:        -   (a) a cured first chemical composition comprising a            polymeric resin at an interface between said polymeric            coating and said polymer or polymer composite material layer            extending at least 2.5 microns in height from the outer            surface of the polymer or polymer composite material layer;        -   (b) a cured second chemical composition comprising the            polymeric resin and an icephobic material addition forming            an exposed outer surface of said non-isotropic article            extending at least 2.5 microns in depth from the exposed            outer surface which, after curing:            -   (i.b) contains an icephobic material addition composed                entirely of solids representing up to 25% by weight of                the cured second chemical composition layer;            -   (ii.b) has a Shore D-Scale Hardness of at least 20,            -   (iii.b) has a sand erosion value according to standard                ASTM G76 at an impingement angle of 90 degrees of less                than 10 mm³/kg;            -   (iv.b) is hydrophobic; and            -   (v.b) has an ice adhesion of less than 200 kPa as                prepared and of less than 350 kPa after 5 icing/deicing                cycles when measured according to ERDC/CRREL Technical                Note 03-4;    -   said non-isotropic article exhibiting:    -   (iii) no failure after being exposed to at least one temperature        cycle according to ASTM B553-71 service condition 1; and    -   (iv) the pull-off strength between the polymer or polymer        composite material layer and an exposed outer surface of the        polymeric coating, according to standard ASTM 4541D is at least        300 psi.

Accordingly, the invention in one exemplary embodiment is directed to anarticle having on its outer surface a polymeric material coating whichis smooth (R_(a)<2.5 microns, preferably <1 micron and more preferably<0.5 microns), hydrophobic and icephobic.

Accordingly, the invention, in another exemplary embodiment is directedto a polymeric coating which adheres well to a metallic or polymericsubstrate and, at and near the exposed outer surface, comprises apolymeric material containing one or more silicone additives, e.g.,modified silicones.

The following further defines the article of the invention:

Substrate Specification:

In one embodiment the base article/substrate the coating is applied tois a metallic material. Typical metals and alloys used comprise at leastone element selected from the group consisting of Al, Co, Cr, Cu, Fe,Mg, Ni, Sn, Ti, W, Zn, and Zr, with alloying additions consisting of B,P, C, Mo, S, and W, and particulate additions consisting of carbides,oxides, nitrides and carbon (carbon nanotubes, diamond, graphite,graphite fibers, graphene).

In another embodiment the base article/substrate the coating is appliedto can also be a polymeric material comprising at least one of:thermosets such as unfilled or filled epoxy, phenolic or melamineresins, polyester resins, urea resins; thermoplastic polymers such asthermoplastic polyolefins (TPOs) including polyethylene (PE) andpolypropylene (PP); polyamides, mineral filled polyamide resincomposites; polyphthalamides, polyphtalates, polystyrene, polysulfone,polyimides; neoprenes; polybutadienes; polyisoprenes; butadiene-styrenecopolymers; poly-ether-ether-ketone (PEEK); polycarbonates; polyesters;liquid crystal polymers such as partially crystalline aromaticpolyesters based on p-hydroxybenzoic acid and related monomers;polycarbonates; acrylonitrile-butadiene-styrene (ABS); chlorinatedpolymers such polyvinyl chloride (PVC); and fluorinated polymers such aspolytetrafluoroethylene (PTFE). Polymers can be crystalline,semi-crystalline or amorphous.

Filler additions can include metals, metal oxides, carbides, carbon(carbon, carbon fibers, carbon nanotubes, diamond, graphite, graphitefibers and graphene), glass, glass fibers, fiberglass, metallized fiberssuch as metal coated glass fibers, mineral/ceramic fillers such as talc,calcium silicate, silica, calcium carbonate, alumina, titanium dioxide,ferrite, mica and mixed silicates (e.g. bentonite or pumice).

Substrates/base articles are made or shaped by any convenientmanufacturing process. It is desirable to suitably prepare a surface ofthe substrates/base article before it receives a coating. Thepretreatment can involve a cleaning step followed by a suitablemechanical or chemical process which increases the surface roughness.

Polymeric Coating Specification:

The polymeric coating contains a curable resin which can be anythermoset resin that can be cured or “set” by crosslinking. Particularlysuitable are epoxy resins including, but not limited to, solid andliquid epoxies from Bisphenol A, Bisphenol F, Diglycidyl Ether ofBisphenol A (DGEBPA), Diglycidyl Ether of Bisphenol F (DGEBPF), Modifiedepoxies including Carboxyl terminated Butadiene acrylonitrile polymer(CTBN) adducted epoxies of DGBPA and DGBPF, and Cresyl Glycidyl Ether orn-Butyl Glycidyl Ether or Phenyl Glycidyl Ether modified epoxy resins ofDGBPA and DGBPF. Preferred polymeric resins are an epoxy resin with anepoxy-equivalent weight (EEW) between 100 and 1,000, preferably between200 and 750.

The polymeric coating can also contain an elastomer such as anyalkadiene polymer, e.g., neoprene rubber; isoprene rubber; butadienerubber, and the like. Modified epoxies containing rubber or siliconeadducts are also suitable. In addition, elastomers can includepolyurethanes, ethylene-propylene rubbers (EPR, sometimes called EPMreferring to an ASTM standard), ethylene-propylene-diene rubber (EPDM)and silicone based elastomers. Preferred rubbers are carboxyl terminatedbutadiene acrylonitrile polymers (CTBN) and/or amine terminatedbutadiene acrylonitrile polymers (ATBN). The polymer coating can beelastomer free (0%), in case elastomers are used its content in anycured layer preferably is kept to no more than 60%, preferably less than50%, more preferably less than 40%, and less than 30% of the weight ofthe curable resin or the total weight of the cured layer. Whenelastomers are present, the elastomer content is preferably at least 5%,more preferably at least 10%, and even more preferably at least 20%. Theelastomer does not need to be present throughout the coating in thedeposition direction, e.g., an elastomer in the bonding layer in contactwith the substrate may be beneficial whereas it is not necessarilyrequired in the outer, icephobic layer, unless it is also the icephobicadditive or it is added to, e.g., reduce the Shore D hardness of theouter layer to render it soft and flexible to enhance the shedding ofice.

The polymeric coating typically contains a curing agent. Any curingagent known in the art is suitable for this purpose. Particularlysuitable are curing agents selected from the group consisting ofamide-type, amine-type and imidazole-type curing agents, moreparticularly imidazole-type curing agents as well as noble metals. Theamount of curing agent is kept to no more than 20%, more preferably lessthan 10%, and less than 7.5% by weight of the cured layer and/or theweight of the resin, e.g., the epoxy resin content, in the formulation.

The polymeric coating can be fiber reinforced. Examples of reinforcingfibers include glass fibers, aramide fibers, carbon fibers, carbonnanotubes, and the like. Other additives can include fluorinatedpolymers such as polytetrafluoroethylene (PTFE) or fluorinated siliconesas well as pigments to provide a coating with any desirable color.

In general, the resin compositions used for forming the polymericcoating can be cured at temperatures below 150° C. For example, curingat about 140° C. for 2 hours, or at 120° C. for 4 hours is generallysufficient to accomplish substantially full curing. Under certaincircumstances, however, e.g., for use in repair and overhaul,compositions which cure at room temperature are desirable.

An annealing step can be added to increase the adhesion between thepolymeric coating layers and between the polymeric coating and theunderlying substrate. The annealing step is a heat treatment step,similar to the curing step, in terms of temperature and duration.

As indicated, the inventive polymeric coating is not homogenous in thedeposition direction but layered and/or graded to enable the outersurface to be icephobic due to the presence of an “icephobic additive”whereas the composition of the polymeric coating near the inner surfaceis optimized for maximizing the bond strength between the organiccoating and the underlying substrate.

In addition to thermosets, polymeric coating can also be formulatedusing thermoplastics such as thermoplastic polyurethanes (TPU).

Icephobic Additive Specification:

Icephobic material additions include paraffins, silicones(polysiloxane), preferably epoxy-modified silicones, fluorinatedsilicones, fluorocarbons, polyurethanes such as polyurethane rubber,fluorinated polyols, polyethers, fluorocarbon elastomers andcombinations thereof. The icephobic addition content preferablyrepresents at least 1%, preferably at least 2.5%, more preferably atleast 5%, more preferably at least 10%, and even more preferably atleast 15% and up to 20% and preferably up to 25% of the weight of thecured layer and/or the weight of the resin, e.g., the epoxy resincontent, in the formulation.

Silicones (polysiloxanes) are particularly preferred additives forachieving icephobic behavior. Silicone elastomers are often referred toas silicone-based polymers that have been vulcanized. Albeit technicallyincorrect, the term “silicone rubber” is often used since it is moredescriptive. Silicone elastomers or silicone rubber materials arereadily available in a hardness ranging from 10 durometer Shore A(extremely soft) to 60 durometer Shore A (firm, medium soft to mediumhard). Properties of silicones are determined by the organic groupsattached to the silicon atoms, and can be fluids, resinous materials orrubbery materials. “Silicones” are typically water-repellent and areused as adhesives, lubricants, hydraulic oils and caulks (sealants). Forthe purpose of increasing icephobicity, silicones with low surfaceenergy and excellent elasticity are desired. As noted above, NuSil™R-2180 from NuSil Technology, LLC, Santa Barbara, Calif., USA, which isa two-part silicone elastomer dispersed in xylene, is a commonly usedcommercially icephobic coating.

Particularly suitable icephobic additives include reactive siliconeswhich are multifunctional silicone pre-polymers with reactive terminalend groups such as epoxy-modified silicones.

Definitions:

The term “substrate” as used herein means a structural product that canbe used as a base for an article.

As used herein, the term “metal matrix composite” (MMC) is defined asparticulate matter embedded in a metallic matrix.

As used herein, the term “filled” or “reinforced” refers to polymerresins which contain fillers embedded in the polymer, e.g., fibers madeof carbon, graphite, carbon nanotubes, graphene, glass and metals;powdered mineral fillers (i.e., average particle size 0.01-25 microns)such as talc, calcium silicate, silica, calcium carbonate, alumina,titanium oxide, ferrite, and mixed silicates. A large variety of filledpolymers having a filler content of up to about 75% by weight or volumeare commercially available from a variety of sources.

As used herein, “prepreg” is an abbreviation for pre-impregnatedreinforcement fabric and/or fiber mats which are commercially availableand used to provide structure and reinforcement for composite articles.The prepreg member is either a dry or wet lay-up component. A dry lay-upis typically a pre-formed structure partially formed prior to beingplaced onto the release layer. A wet lay-up consists of placing a fabricor fibers onto the release layer, whereupon a liquid epoxy compositionis subsequently poured onto the fibers to impregnate the fibers. Apartial curing step may be applied to the prepreg member wherenecessary.

As used herein, the term “coating” means a deposit layer applied to partor all of an outer surface of a substrate.

As used herein, the term “coating thickness” or “layer thickness” refersto the depth in the deposition direction and typical thicknesses exceed25 microns, preferably 100 microns.

The term “bonding layer” as used herein refers to an intermediate layerdirectly adjacent to the substrate and between the substrate and theoutermost coating layer exposed to the elements of the article ofmanufacture.

As used herein, “exposed surface” and “outer surface” refer to allaccessible surface area of an object accessible to the atmosphere and/ora liquid. The “exposed surface area” refers to the summation of all theareas of an article accessible to a liquid.

As used herein, “surface roughness”, “surface texture” and “surfacetopography” mean a regular and/or an irregular surface topographycontaining surface structures. These surface irregularities/surfacestructures combine to form the “surface texture”.

As used herein the term “smooth surface” is characterized by a surfaceroughness (R_(a)) less than or equal to 1 micron.

The term “epoxy” or “epoxy resin” as used herein refers to a flexibleusually thermosetting resin made by copolymerization of an epoxide withanother compound having two hydroxyl groups and used predominately incoatings and adhesives.

The term “curable epoxy resin” as used herein refers to resins relyingon a ring opening reaction of the epoxy functional group topolymerize/cross-link.

The term “polyurethane” as used herein refers to polymers composed oforganic units joined by carbamate (urethane) links. While mostpolyurethanes are thermosetting polymers that do not melt when heated,thermoplastic polyurethanes are also available. Polyurethane polymersare commonly formed by reacting a di- or tri poly-isocyanate with apolyol. Since polyurethanes contain two types of monomers, whichpolymerize one after the other, they are classified as alternatingcopolymers.

The term “epoxy equivalent weight” or “EEW” as used herein is used forformulating epoxy adhesive compositions. EEW is defined as the weight ofa resin in grams that contains one equivalent of epoxy.

The term “adhesion promoters” as defined herein contains additives whichimprove adhesion and strongly adsorb onto the surface of the substrate.Ideally, the adsorption is so strong that rather than being a physicaladsorption it has the nature of a chemical bond. Adhesion promoters actat the interface between an organic polymer and an inorganic substrateto enhance adhesion between the two materials.

The term “elastomer” as used herein refers to amorphous polymersmaintained above their glass transition temperature, so thatconsiderable molecular reconfirmation, without breaking of covalentbonds, is feasible. At ambient temperatures, elastomers are thusrelatively soft and deformable. Examples of elastomers includepolyisoprene (natural and synthetic “rubbers), polyurethanes andsilicones (silicone “rubbers”).

The term “rubber” as used herein refers to any polymer comprising analkadiene (isoprene) as one of its monomers.

The term “silicones”, also known as “polysiloxanes” as used hereinrefers to polymers that include any synthetic compound made up ofrepeating units of siloxane or (—Si—O—Si—O—)_(n), which is a chain ofalternating silicon atoms and oxygen atoms, combined with carbon,hydrogen, and sometimes other elements.

The term “epoxy modified silicone” as used herein refers to reactivesilicones, which can be used to create epoxy-silicone hybrids of highdurability. By combining the advantages of epoxies (e.g., good adhesion,high abrasion resistance and good mechanical strength but poor UVstability and low hydrophobicity) and silicones (e.g., good flexibilityand elongation, excellent UV stability and hydrophobicity but poorabrasion resistance and adhesion) excellent properties can be obtained.

The term “curing” as used herein refers to cross-linking process thatresults in a three-dimensional molecular polymeric structure.

The term “curable resin” refers to a resin composition that can be curedby crosslinking.

The term “substantially fully cured” refers to a curable resin that hasbeen subjected to a heat treatment at a temperature that is high enough,and for a time that is long enough, to result in a completion of thecrosslinking process.

As used herein the term “curing agent” refers to a substance that isused to harden a material. It is typically applied to polymers tofacilitate the cross-linking and bonding of its molecular components.

The term “cross-link” as used herein refers to a bond that links onepolymer chain to another. These links may take the form of covalentbonds or ionic bonds and the polymers can be either synthetic polymersor natural polymers.

As used herein, the term “contact angle” or “static contact angle” isreferred to as the angle between a static drop of deionized liquid waterand a flat and horizontal surface upon which the droplet is placed and,unless otherwise indicated, is determined at room temperature.

As used herein the term “hydrophilic” is characterized by the contactangle for water obtained at room temperature of less than 90°, whichmeans that the liquid water droplet wets the surface.

As used herein the term “hydrophobic” or “wetproof” is characterized bythe contact angle for liquid water obtained at room temperature ofgreater than 90°, which means that the water droplet does not wet thesurface.

As used herein, “super-hydrophobic” refers to a contact angle fordeionized water at room temperature equal to or greater than 150° and“self-cleaning” refers to a tilt angle of equal to or less than 5°.

As used herein, the term “tilt angle” or “roll-off angle” means thesmallest angle between a surface containing a water droplet and thehorizontal surface at which the droplet commences to and keeps rollingoff at room temperature.

As used herein, the term “icephobic” or “pagophobic” means the iceadhesion strength according to the ice adhesion test described by theU.S. Army Engineer Research and Development Center is less than 300 kPa.

As used herein the term “ice adhesion test” is the one described by theU.S. Army Engineer Research and Development Center, Hanover, N.H., USA,in the ERDC/CRREL Technical Note 03-4 (October 2003).

The term “pull-off strength” as used herein refers to the strength ofthe adhesive bond between the layered metal construct and the bondinglayer (or between the bonding layer and the substrate, whichever islower). Pull-off strength is measured according to standard ASTM 4541D;its dimension is [force]/[length]. Test results are reported in psiunits or, more properly, MPa (1 psi=0.0069 MPa).

As used herein “thermal cycling performance” is characterized by theANSI/ASTM specification B604-75 section 5.4 Test (Standard RecommendedPractice for Thermal Cycling Test for Evaluation of ElectroplatedPlastics ASTM B553-71). In this test the samples are subjected to athermal cycle procedure as indicated in Table 1. The sample is held atthe high temperature for an hour, cooled to room temperature and held atroom temperature for an hour and subsequently cooled to the lowtemperature limit and maintained there for an hour.

TABLE 1 Thermal Cycle Procedure Information Service Condition High LimitLow Limit 1 (mild) 60° C. −30° C. 2 (moderate) 75° C. −30° C. 3 (severe)85° C. −30° C. 4 (very severe) 85° C. −40° C.

The term “Shore hardness” as used herein refers to a measure of theresistance a material has to indentation. There are three differentShore Hardness scales (00, A and D) for measuring the hardness ofdifferent typically polymeric materials (supersoft gels, soft elastomersand rigid plastics).

As used herein the term “sand erosion” refers to a material loss of anarticle caused by sand impingement. The ASTM G76 “Standard Test Methodfor Conducting Erosion Tests by Solid Particle Impingement Using GasJets” measures the material loss of a surface caused by gas-entrainedsolid particle impingement with sand from a jet-nozzle This test methodcan be used for determining erosion for various particle sizes,velocities, attack angles, environments, etc. The ASTM G76 Conditionsused in this specification include an impingement angle as indicated,Al₂O₃ powder abrasive with an average particle size of 50 μm, a gaspressure of 20 psi, a blast duration of 1 min, a particle feed rate of10.0±2.0 g/min, a distance between the test coupon and nozzle tip of50±1 mm, compressed nitrogen as the carrier gas, an ambient testtemperature, and flat sample panels. The person skilled in the art willknow that the erosion rate under otherwise similar conditions depends onthe impingement angle. While an impingement angle of 90° is commonlyused to characterize various materials, depending on the ductility ofthe substance being tested, the erosion rate can vastly change with theimpingement angle, i.e., ductile materials typically have the lowesterosion rate at 90°, whereas brittle materials experience the lowesterosion values at very low impingement angles as indicated, e.g., in J.Vite-Torres et. al., “Solid Particle Erosion on Different MetallicMaterials” Tribology in Engineering, Chapter 5, Intech Open Science,2013.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be appreciated uponreference to the following drawings, in which:

FIGS. 1 and 3 are graphs showing the wetting angle, ice adhesion, ShoreD hardness as well as the sand erosion properties for impingement anglesof 90° and 30° for various polymeric coatings.

FIGS. 2 and 4 are graphs showing the pull-off adhesion strength ofselected polymeric coatings applied to various substrates.

DETAILED DESCRIPTION OF THE INVENTION

It is well known that the formation, adhesion, and accumulation of ice,snow, frost, glaze, rime, or their mixtures can cause severe problemsfor solar panels, wind turbines, aircrafts, heat pumps, power lines,telecommunication equipment, as well as land vehicles and marinevessels. These problems generate safety hazards and can result infailure. To address these issues, the fundamentals of interfaces betweengases, liquids and solids and solid surfaces at low temperatures need tobe taken into account and various approaches to form “icephobic”(pagophobic) surfaces have been proposed. As the person skilled in theart knows different properties may be required to prevent the formationand adhesion of ice, snow, glaze, rime, and frost.

Icephobicity is the ability of a solid surface to repel ice or preventice formation due to a certain topographical structure and/or chemicalcomposition of the surface. Icephobic surfaces in this specification aredefined by an ice adhesion strength of typically <350 kPa, preferably<300 kPa, and more preferably <200 kPa. Slippery, liquid infused poroussurfaces (SLIPS) have been proposed for reducing ice adhesion to valuesas low as 10 kPa, however, after a few icing-deicing cycles, iceadhesion gradually increases to over 200 kPa. Furthermore, themechanical durability of SLIPS surfaces is typically poor. Many otherapproaches have been proposed as well which are capable of reducing theicephobicity, however, achieving and maintaining both icephobicity andlong-term durability remains a challenge.

In order to enhance icephobicity, the exposed surface needs to generallyexhibit poor adhesion characteristics, however, in most of thecommercial applications the icephobic material layer is applied as ahomogeneous coating onto a suitable substrate. As a consequence, the“inner surface” of such icephobic coatings, e.g., the surface contactingthe underlying substrate, exhibits poor adhesion particularly whencontaining liquid icephobic additives. Delamination and flaking of thepolymeric coating at the interface with the underlying substratefrequently limit its durability even if the coating is formulated to behard and strong.

In this specification therefore the inventors of the present disclosurepropose to form anisotropic, icephobic coatings by minimizing adhesionon the exposed outer surface while maximizing adhesion on the innersurface to achieve a good bond at the interface with the underlyingsubstrate. This is achieved by modifying the composition of theicephobic material coating in the deposition direction through layeringand/or gradually modulating the chemical composition. In its simplestform the coating contains two layers of different composition.

Epoxies are known for achieving excellent adhesion to a number ofsubstrates including metals due to their polar nature and their abilityto create chemical bonds to the surface upon cure. Hard and strong dueto high crosslinking, cured epoxies can bear loads and resist wearcaused by abrasion over the long term. This good mechanical strength,however, at high crosslink density flexibility decreases and epoxies areknown for cracking due to their brittleness and inability to dissipatestresses. Cured epoxies layers require the addition of icephobicmaterial additives to notably reduce ice-adhesion strength.

Similarly, polyurethanes are commonly employed as coatings forpreventing corrosion of metal articles and to make a variety ofmaterials more durable. Polyurethanes are extremely resilient substancesand their mechanical properties can be easily manipulated by optimizingtheir compositions.

Silicones exhibit high elongation and flexibility that enable thedissipation of stresses and applied energy. Their strong Si—O bond andhigh surface energy make silicones resistant to atmospheric or chemicalattack and less susceptible to degradation from sunlight, water uptakeand ultraviolet (UV) light, thereby providing hydrophobic and icephobicproperties for extended periods of time.

The inventors of the present disclosure have discovered that combiningthe adhesion, abrasion resistance and mechanical strength of epoxieswith the thermal stability, flexibility and icephobicity of silicones inhybrid systems yields strong and flexible materials with higherresistance to cracking in harsh environments while providing hydrophobicand icephobic properties. Starting out at the metallic or polymericsubstrate surface representing the inner coating surface by applying apolymeric coating with a high epoxy content, preferably beingsubstantially free of silicones or having a low silicone content assuresa good bond between the polymeric coating and the underlying substrate.Then, as the thickness of the polymeric coating increases, the siliconecontents in the deposition direction towards the outer exposed surfacecan be gradually or stepwise increased. Consequently, a polymericcoating is formed ending up with the highest silicone content at theouter, exposed surface, thus providing for an anisotropic polymericcoating which maintains excellent adhesion to the substrate surfacewhile being icephobic on the exposed, outer surface. In one preferredembodiment the same epoxy resin is used throughout the coating, i.e., inboth the bonding and the icephobic layer.

Similarly, substituting polyurethanes based coatings for epoxies usingotherwise the same approach can yield durable coatings with excellenticephobic properties and high adhesion to the underlying substrates.

This invention relates to articles comprising durable, icephobiccoatings. In its broadest aspect the present invention relates toarticle of manufacture comprising:

-   -   (i) a substrate comprising a metallic or polymeric material        having an outer surface in direct contact with    -   (ii) an anisotropic polymeric coating which        -   (a) on the interface between said polymeric coating and said            substrate contains a substantially fully cured organic resin            and is substantially free of icephobic additives; and        -   (b) on its exposed outer surface contains the same            substantially fully cured organic resin and furthermore            contains one or more solid icephobic additives.

In one preferred embodiment the substrate comprises a metallic material.Examples of suitable metallic materials include metals and alloys ofaluminum, cobalt, magnesium, steel, nickel and titanium. The substratecan be an isotropic, layered or graded metallic construct comprising oneor more continuous metal layers wherein at least one of the continuousmetal layers is a microcrystalline and/or amorphous metal layer or agrain-refined layer having a grain size below 5,000 nm.

In another preferred embodiment the substrate comprises a polymer orpolymer composite. Suitable polymers include any known thermoplastic orthermoset. Suitable polymer composites can contain a material selectedfrom the group consisting of carbon, carbon fibers, graphite, graphitefibers, carbon nanotubes and graphene. Other additions such as glass,glass fibers, as well as inorganic and organic fibers are contemplatedas well.

The polymeric coating applied to the substrate is anisotropic in thedeposition direction, e.g., layered or compositionally graded. Thecomposition of the polymeric coating in contact with the substrate andnear the interface with the substrate is chosen to maximize the adhesionstrength between the polymeric coating and the substrate. In contrast,the composition of the exposed outer surface of the polymeric coating ischosen to maximize erosion performance and icephobic properties. Thetransition of the composition of the polymeric coating from the “bondingsurface” to the “icephobic surface” can be gradual, e.g., by changingthe chemical composition from an epoxy or polyurethane rich and siliconefree to an epoxy or polyurethane and silicone containing outer surfaceproviding a graded polymer layer. Alternatively, distinct layers ofvarious compositions can be applied to transition from an epoxy orpolyurethane rich, silicone-free to an epoxy or polyurethane andsilicone containing outer surface. In the case of layering, at aminimum, two distinct polymer layers are applied; however, a multilayerlaminate with a total of up to 100 sublayers can be applied. Combinationof layered and graded sublayers is also included in the scope of thisinvention.

In an alternative embodiment the anisotropic polymeric coating is notdirectly applied to the substrate but formed independently and providedwith an adhesive film or an adhesive tape in contact with the bondinglayer and the exposed adhesive film is protected by a release liner.Before use, the release liner is removed and the coating is applied tothe substrate in a way that the adhesive film forms an intermediatelayer between the substrate and the anisotropic polymeric coating. Theadhesive film can be epoxy based providing high adhesive strength andpreferably cures at or near room temperature. Other options includerubber-, silicone- or acrylic-based adhesive tapes which can also bepressure sensitive adhesives. In one preferred embodiment the adhesivelayer replaces the bonding layer and is applied directly onto theicephobic layer. Such an approach can be used to apply the icephobiccoating conveniently onto any part to render it icephobic, it isparticularly suitable for use in repair and overhaul, e.g., where thereis a requirement to patch eroded or deteriorated sections of thecoating.

The present invention is based on the discovery that, in the case ofapplying an icephobic organic coating to a metallic or polymericsubstrate surface, an isotropic coating does not readily achieve thedesired overall performance as either adhesion to the underlyingsubstrate or icephobic properties or both are unduly compromised whichcan have a significant effect on performance and durability. Monolithiccoatings optimized for icephobicity were found to have rather pooradhesion to the substrate materials of interest and perform poorly onextended durability tests such as sand or rain erosion frequentlyresulting in part or the entire coating flaking off the underlyingsubstrate surface resulting in premature failure.

Typically, the polymeric coating according to this invention uses thesame ingredients throughout, e.g., the same curable epoxy orpolyurethane resin, the same curing agent formulation etc. termed “basiccoating formulation”. The main difference in the chemical formulationsnear the “bonding surface” when compared to near the “icephobic surface”is, that, in the case of the “near bonding layer surface” the coatingcomprises merely the “basic coating formulation” with the optionaladdition of adhesion promoters, and elastomers. In contrast, “near theouter icephobic surface” the coating comprises the “basic coatingformulation” with the addition of between 1 and 20 wt % of an “icephobicadditive” with the optional addition of other additives, including, butnot limited to, abrasive materials to enhance the hardness and erosionresistance of outer and near-outer surface and pigments to achieve anydesired color. Consequently, the chemical composition throughout thepolymeric coating is similar assuring excellent adhesion of sublayers,if any, to each other and excellent overall cohesive strength of theentire coating. This approach is far superior to, e.g., applying adistinct polymer primer to the substrate followed by the deposition of adistinct and unrelated, icephobic polymeric coating, which can lead todelamination of the “unrelated” polymer layers during use.

Accordingly, the polymeric coating applied to the metallic or polymericmaterial substrate is anisotropic and comprises, on the surface incontact with the substrate material, a “bonding layer” comprising acurable resin preferably free of silicones and, on the exposed outersurface an “icephobic layer” which, in addition to the same curableresin, contains at least 1 and up to 25 weight percent of a modifiedsilicone.

Both, the bonding layer and the outer icephobic layer have a thicknessof at least 5 microns, preferably at least 25 microns and morepreferably at least 50 microns.

The bonding layer is deposited directly onto the substrate material andsubsequently typically at least partially cured and another cure isperformed after the icephobic top coat is applied. Depending on thenumber of sublayers the total number of curing steps involved is atleast 2 but as many as 10+ curing cycles can be used, depending on thenumber of sublayers applied and the final properties desired.

In another aspect the invention provides a process for coating anarticle, said process comprising the steps of:

-   -   providing an article of manufacture having an outer surface, or        a predetermined portion thereof, comprised of a metallic or        polymeric substrate material;    -   coating the outer surface of the substrate material, or a        predetermined portion thereof, with a curable polymeric resin of        a first composition;    -   substantially partially or fully curing the curable polymeric        resin of the first composition to form a bonding layer;    -   coating the outer surface of the bonding layer with a polymeric        resin of a second composition comprising an icephobic additive;    -   substantially fully curing the curable polymeric resin of the        second composition to form a durable icephobic exposed outer        surface;

In another aspect the invention provides a process for applying aprepreg coating to an article, said process comprising the steps of:

-   -   providing an article of manufacture having an outer surface, or        a predetermined portion thereof, comprised of a metallic or        polymeric substrate material;    -   independently forming and substantially partially or fully        curing a curable polymeric resin of the second composition        comprising an icephobic additive to form a durable icephobic        exposed outer surface;    -   optionally applying onto the second composition layer a curable        polymeric resin of a first composition and optionally partially        or fully curing the curable polymeric resin of the first        composition;    -   applying onto the first composition layer, if present, an        adhesive layer and optionally partially or fully curing the        multilayer construct;    -   optionally applying onto the adhesive layer a release liner;    -   upon use removing the optional release liner from the multilayer        construct and applying it to the metallic or polymeric substrate        material so that the icephobic layer of the second composition        becomes the exposed outer surface.

Such processes result in achieving a very strong bond between the outerexposed coating surface and the underlying core substrate and theprocesses can be used to manufacture articles in which strong adhesionbetween the exposed polymeric coating and the underlying substrate isdesired or necessary. In addition, the organic coating sublayers areformulated and/or processed to maximize cohesive strength within thepolymeric coating sublayers themselves while achieving excellentadhesion between the sublayers as well. The processes are particularlysuited for the manufacture of durable articles that require high waterrepellency, icephobicity, as well as abrasion resistance and flexural,tensile, torsional, impact and/or fatigue strength, such as required forsporting goods, automotive parts, aircraft components, buildingmaterials, industrials components exposed to the elements; and the like.

It is desirable to pretreat the substrate surface before it receives thepolymer coating. The pretreatment can comprise mechanical abrasionand/or etching. Etching can be, e.g., accomplished with permanganate orsulfochromic chemical etch, or with a plasma etch.

The compositions comprising the curable resin can, for example, beapplied by spraying. For this purpose, the composition desirably uses asolvent in a sufficient amount to obtain the viscosity suitable forspraying. It has been found that preferred solvents have a boiling pointof less than 150° C., preferably less than 100° C. The importance of theboiling point of the solvent is related to the need to have the filmsubstantially fully cured. It is important that, after curing, thepolymeric coating has substantially no dissolved solvents and does notcontain any liquids.

When applied by spraying, both the bonding layer and the icephobic layerare generally applied at about 3 to 20 mg/cm², preferably from 5 to 15mg/cm². Depending on the thickness desired, it may be advantageous toapply each composition in two or more sprayed layers, with a partialcuring (for example 30 minutes at 140° C. or 90° C.) betweenapplications.

After applying the bonding layer and prior to depositing the icephobicouter layer, the bonding layer can be partially or fully cured as wellas suitably pretreated. This pretreatment can comprise mechanicallyroughening and/or etching. Suitable etching processes include chemicaletching processes and/or plasma etching.

When applying the icephobic layer, the spraying process described forapplying the bonding layer is essentially replicated. Alternativeprocesses can be used as well including, but not limited to, painting,doctor blading and screen printing.

While building up the entire polymeric coating, various curing steps canbe performed and repeated, alternatively partial curing steps can beemployed or the curing step can be deferred until the entire coating hasbeen deposited.

As the person skilled in the art of organic coatings would appreciate,the organic coating can be applied to the core substrate in an automatedproduction line where, e.g., the substrate to be coated passes from onespray booth to the next with optional partial or total curing and/orsurface treatments in between. A multilayer laminate coating can beproduced, e.g., changing the composition of the “paint” in each spraystation to achieve a “stepwise” transition from the bonding layercomposition to the icephobic surface composition. While the organiccoating of this invention relies on at least two distinct layers, amultitude of layers, such as 5, or 10 or even more transitional layerscan be incorporated between the inner surface (in contact with thesubstrate) and the outer (exposed) surface of the organic coating.Similarly, rather than having a multilayer laminate with distinctchemical compositions, a gradual change in the composition can beaffected in the coating deposition direction and combinations of layeredand graded layers are within the scope of this invention as well.

It is also possible to provide the icephobic polymer layer infreestanding form or supported such as a “pre-preg” as described above.The bonding layer film or pre-preg used in this process can befabricated from the liquid epoxy formulation using standard industrypractices used for fabricating thin film polymeric adhesive films andpre-pregs from solvent bearing formulations. The release linerprotecting the adhesive film/transfer tape is removed before use and theanisotropic polymeric coating comprising an icephobic layer, a bondinglayer and an additional adhesive layer or, alternatively, the adhesivelayer is replacing the bonding layer altogether, is then applied to thesubstrate. For instance, the icephobic layer can be applied, e.g.,sprayed onto the exposed surface of an adhesive tape which may have arelease liner on the opposite side.

Articles or coatings made according to the process of this inventionfind use in a variety of applications requiring improved durabilitywhile retaining enhanced hydrophobic and icephobic properties.

EXAMPLES

The following is a description of Working Examples illustrating thebenefits of the present disclosure, specifically the formulation ofvarious polymeric coatings and methods of applying and curing thecoatings as well as selected properties including the icephobicproperties (Working Example I and III) as well as adhesion propertiesusing a variety of substrates (Working Example II and IV).

Example I: Epoxy (EP) Coating Formulations, Application and SelectedProperties

Various coating formulations were investigated as follows: Nusil™, acommercial product (two-part silicone elastomer dispersed in xylene)available from NuSil™ Technology LLC, Carpinteria, Calif. 93013, USA,which is widely recognized for its icephobic properties as indicatedabove, was applied to selected substrates by doctor blade whereas thevarious inventive epoxy-based coatings were applied to the substrates byspraying using a gravity feed type, HVLP (high volume-low pressure)epoxy spray gun operated at 60 psi. In-house paint formulations areprovided in Table 2 below. The coated substrates were subsequently curedin a furnace at 140° C. for 2 hours, except for Nusil™, which was curedaccording to the manufacturer's specifications using four temperaturesand durations as follows: RT/30 min, 75° C./45 min, and 150° C./135 min.Total target loading for each sample was 8-12 mg/cm² (thickness: 75-100microns). If multiple layers were applied, e.g., an icephobic layer ontop of the bonding layer, the thickness and loading for each layer wasreduced to maintain the overall coating thickness/loading target and acuring step was performed after each layer. After curing, the exposedsurface of all samples was smooth and had a surface roughness R_(a)<1μm.

Cured coating samples produced were characterized as indicated in Table3. For ease of comparison, the data of the various tests depicted inTable 3 are also shown in FIG. 1 , namely the wetting angle, the iceadhesion according to the test described by the U.S. Army EngineerResearch and Development Center, Hanover, N.H., USA, in the ERDC/CRRELTechnical Note 03-4 (October 2003), the sand erosion value atimpingement angles of 90° and 30° according to ASTM G76, and the Shore Dhardness.

TABLE 2 In-house Epoxy (EP) Paint Formulations Investigated. Epoxy ResinIcephobic Epoxy Bonding Layer Resin Top Layer Formulation [g]Formulation [g] Epoxy Resin (EEW = 550) 100 100 Elastomer 60 0 CuringAgent 5 5 Adhesion Promoter 8 0 Additives 15 0 Icephobic Additive:Epoxide 0 7.5 Functional Silicone Pre-Polymer Silicone Oil 0 0 Solvent240 120

The results indicate that all coating surfaces are hydrophobic, that anicephobic additive such as a silicone is required to achieve a lowicephobicity value on the first cycle (Nusil™ and silicone top layercontaining coatings) and that, after repeated icing/deicing cycles seventimes, the ice adhesion of the samples containing the solid siliconeremain largely unchanged (modified silicone top layer containingcoatings).

The data also reveal that the sand erosion values at 90° impingementwith icephobic additions increase compared to the additive freeformulation of the bonding layer, however, the erosion values at animpingement angle of 30° experience similar mass losses for all samples.In addition, the Shore D hardness values of the coatings containingepoxy resins are significantly higher than the one for Nusil™.

TABLE 3 Selected Coating Properties Two-Layers Elastomer-Single/Homogenous Layer Free 6.7% Bonding Silicone Silicone LayerContaining Free plus Outer Epoxy Epoxy Exposed Layer Layer Surface(Icephobic (Bonding Icephobic Nusil ™ Layer) Layer) Layer ExposedSurface <1 <1 <1 <1 Roughness, R_(a) [μm] Wetting Angle [°] 119 123 115123 Cycle 1: 150 110 472 110 Ice Adhesion [kPa] Cycle 7: 305 110 691 110Ice Adhesion [kPa] Sand Erosion Rate 4.1 4.9 1.9 4.9 @ 90° [mm³/kg] SandErosion Rate 2.1 1.8 1.9 1.8 @ 30° [mm³/kg] Shore Hardness 40 (scale A)70-75 70-75 70-75  0 (scale D) (scale D) (scale D) (scale D)

Example II: Article Characterization

A number of substrates were selected (4×4 inch panels) for thisinvestigation, namely Al, Ti, stainless steel, carbon fiber reinforcedcomposite and Nylon. The smooth substrates (R_(a)<1 μm) weremechanically abraded with ultra-fine (500 grit) sand paper to a uniformfinish, then cleaned and degreased by wiping with a suitable solvent andvarious coatings described in EXAMPLE I were applied as follows: (i)Nusil™, (ii) an epoxy based, silicone-free, bonding layer, (iii) anepoxy based icephobic layer and (iv) a two layer coating comprising theepoxy based bonding layer on the substrate followed by the epoxy basedicephobic outer exposed layer.

TABLE 4 Adhesion Property Evaluation Single/Homogenous Layer 2-Layers6.7% Silicone Elastomer-Free Silicone Free Bonding Layer ContainingEpoxy plus Outer Epoxy Layer Layer Exposed (Icephobic (Bonding SurfaceNusil ™ Layer) Layer) Icephobic Layer Pull Off Adhesion 228 750 13221239 Strength from Titanium [psi] Pull Off Adhesion 213 691 1344 1337Strength from Aluminum [psi] Pull Off Adhesion 201 471 1411 1420Strength from Stainless Steel Grade 304 [psi] Pull Off Adhesion 210 8721846 1887 Strength from Carbon Fiber Composite [psi] Pull Off Adhesion247 428 942 945 Strength from Nylon [psi]

The adhesion between the top/exposed surface of any coating and the basesubstrate was measured by the “pull-off strength” according to standardASTM 4541D. For ease of comparison, the data of the various samples aredepicted in Table 4 are also shown in FIG. 2 . The data reveal thatlayers with the icephobic additive (Nusil™ and icephobic layer) applieddirectly to the varies substrates have the lowest bond strength, whereasthe icephobic additive-free bonding layer and the two-layer structurecomprising a bonding layer in direct contact with the substrate and anouter, exposed icephobic layer results in an overall pull-off strength,at times, that exceeded 800 psi as well as 1,000 psi. Articlescontaining the Nusil™ coating, in all instances, delaminated at theinterface of the substrate and the coating. Articles containing abonding layer in contact with the various substrates, in all instances,showed both signs of delamination as well as cohesive failure, revealingthe impressive strength of these samples.

These results indicate that an anisotropic coating (bondinglayer+icephobic layer) provides a superior article with excellentdurability, bond strength and long lasting icephobic properties.

Example III: Polyurethane (PU) Coating Formulations, Application andSelected Properties

A thermoset polyurethane formulation which cures at room temperaturewith and without the addition of a modified silicone were applied tovarious substrates by spraying using a gravity feed type, HVLP (highvolume-low pressure) spray gun operated at 60 psi and compared to theNusil™ commercial product described above. In-house paint formulationsare provided in Table 5 below. Total target loading for each sample was8-12 mg/cm² (thickness: 75-100 microns). After curing, the exposedsurface of all samples was smooth and had a surface roughness R_(a)<1μm.

Cured coating samples produced were characterized as indicated in Table6. For ease of comparison, the data of the various tests depicted inTable 6 are also shown in FIG. 3 , namely the wetting angle, the iceadhesion according to the test described by the U.S. Army EngineerResearch and Development Center, Hanover, N.H., USA, in the ERDC/CRRELTechnical Note 03-4 (October 2003), the sand erosion value atimpingement angles of 90° and 30° according to ASTM G76, and the Shore Dhardness.

TABLE 5 In-house Polyurethane (PU) Paint Formulations Investigated. PUBonding Layer Icephobic PU Resin Formulation [g] Layer Formulation [g]Thermoset Polyurethane 100 100 Elastomer 0 0 Curing Agent 100 100Adhesion Promoter 0 0 Additives 0 0 Icephobic Additive: 0 10 EpoxideFunctional Silicone Pre-Polymer Silicone Oil 0 0 Solvent 200 200

TABLE 6 Selected Coating Properties Single/Homogenous Layer Two-Layers4.8% PU Bonding Silicone Silicone Layer plus Containing Free PU OuterExposed PU Layer Layer Surface (Icephobic (Bonding Icephobic PU Nusil ™Layer) Layer) Layer Exposed Surface <1 <1 <1 <1 Roughness, R_(a) [μm]Wetting Angle [°] 119 120 120 120 Cycle 1: 150 137 363 137 Ice Adhesion[kPa] Cycle 5: 305 133 532 133 Ice Adhesion [kPa] Sand Erosion Rate 4.14.4 4.4 4.4 @ 90° [mm³/kg] Sand Erosion Rate 3.5 3.5 3.5 3.5 @ 45°[mm³/kg] Sand Erosion Rate 2.1 2.1 2.1 2.1 @ 30° [mm³/kg] Shore Hardness40 (scale A) 20 20 20  0 (scale D) (scale D) (scale D) (scale D)

The results indicate that all coating surfaces are hydrophobic, that anicephobic additive such as a silicone is required to achieve a lowicephobicity value on the first cycle (Nusil™ and silicone top layercontaining coatings) and that, after repeated icing/deicing cycles fivetimes, the ice adhesion of the samples containing the solid siliconeremain largely unchanged (modified silicone top layer containingcoatings).

The data also reveal that the sand erosion at all angles of impingementwith are similar for all samples. In addition, the Shore D hardnessvalues of the coatings containing PU resins are significantly higherthan the one for Nusil™.

Example IV: Article Characterization

A number of substrates were selected (4×4 inch panels) for thisinvestigation, namely Al, Ti, stainless steel, carbon fiber reinforcedcomposite and Nylon. The smooth substrates (R_(a)<1 μm) weremechanically abraded with ultra-fine (500 grit) sand paper to a uniformfinish, then cleaned and degreased by wiping with a suitable solvent andvarious coatings described in EXAMPLE III were applied as follows: (i)Nusil™, (ii) a thermoset polyurethane based, silicone-free, bondinglayer, (iii) an thermoset polyurethane based icephobic layer and (iv) atwo layer coating comprising the thermoset polyurethane based bondinglayer on the substrate followed by the thermoset polyurethane basedicephobic outer exposed layer.

The adhesion between the top/exposed surface of any coating and the basesubstrate was measured by the “pull-off strength” according to standardASTM 4541D. For ease of comparison, the data of the various samples aredepicted in Table 7 are also shown in FIG. 4 . The data reveal thatlayers with the icephobic additive (Nusil™ and icephobic layer) applieddirectly to the varies substrates have the lowest bond strength, whereasthe icephobic additive-free bonding layer and the two-layer structurecomprising a bonding layer in direct contact with the substrate and anouter, exposed icephobic layer results in an overall pull-off strengththat exceeded 1,000 psi. Articles containing the Nusil™ coating, in allinstances, delaminated at the interface of the substrate and thecoating. Articles containing a bonding layer in contact with the varioussubstrates, in all instances, showed both signs of delamination as wellas cohesive failure, revealing the impressive strength of these samples.

These results indicate that an anisotropic coating (bondinglayer+icephobic layer) provides a superior article with excellentdurability, bond strength and long lasting icephobic properties.

TABLE 7 Adhesion Property Evaluation Single/Homogenous Layer 2-Layers4.8% Silicone Silicone PU Bonding Containing PU Free PU Layer plus OuterLayer Layer Exposed Surface (Icephobic (Bonding Icephobic PU Nusil ™Layer) Layer) Layer Pull Off Adhesion 228 1315 1423 1425 Strength fromTitanium [psi] Pull Off Adhesion 213 1298 1455 1420 Strength fromAluminum [psi] Pull Off Adhesion 201 1024 1224 1210 Strength fromStainless Steel Grade 304 [psi] Pull Off Adhesion 210 1677 1685 1650Strength from Carbon Fiber Composite [psi] Pull Off Adhesion 247 8921192 1200 Strength from Nylon [psi]

VARIATIONS

The foregoing description of the invention has been presented describingcertain operable and preferred embodiments. It is not intended that theinvention should be so limited since variations and modificationsthereof will be obvious to those skilled in the art, all of which arewithin the spirit and scope of the invention.

The invention claimed is:
 1. A cured polymeric coating which iscompositionally graded or layered to be anisotropic in the depositiondirection, the cured polymeric coating comprising: a cured firstchemical composition extending at least 2.5 microns in height from anouter surface of an underlying substrate; (ii) a cured second chemicalcomposition having a different chemical composition than the cured firstchemical composition, the cured second chemical composition comprising apolymeric resin and an icephobic material addition forming an exposedouter surface extending at least 2.5 microns in depth from the exposedouter surface, wherein the cured second chemical composition: a)contains the icephobic material addition representing up to 25% byweight of the cured second chemical composition; b) has a sand erosionvalue according to standard ASTM G76 at an impingement angle of 90degrees of less than 10 mm³/kg; and c) has an ice adhesion value of lessthan 300 kPa as prepared when measured according to ERDC/CRREL TechnicalNote 03-4; and said cured polymeric coating has a total thickness of atleast 10 microns, and the pull-off strength between the underlyingsubstrate and the exposed outer surface of the cured polymeric coating,according to standard ASTM 4541D, is at least 250 psi.
 2. The curedpolymeric coating according to claim 1, wherein the cured polymericcoating exhibits no failure after being exposed to at least onetemperature cycle according to ASTM B553-71 service condition
 1. 3. Thecured polymeric coating according to claim 1, wherein the curedpolymeric coating at room temperature is free of liquids.
 4. The curedpolymeric coating according to claim 1, wherein the pull-off strengthbetween the underlying substrate and the exposed outer surface of thecured polymeric coating, according to standard ASTM 4541D, is at least300 psi.
 5. The cured polymeric coating according to claim 1, whereinboth the cured first chemical composition and the cured second chemicalcomposition, prior to curing, comprise one or more curable epoxy resinswith an EEW number of between 100 and
 750. 6. The cured polymericcoating according to claim 1, wherein the cured first chemicalcomposition forms the inner surface contacting the underlying substrate,and said inner surface is rough with a surface roughness of R_(a)>1micron; and the cured second chemical composition forms the exposedouter surface exposed to atmosphere; and said exposed outer surface issmooth with a surface roughness R_(a)<1 micron.
 7. The cured polymericcoating according to claim 1, wherein the cured second chemicalcomposition has an ice adhesion value of less than 500 kPa after 5icing/deicing cycles when measured according to ERDC/CRREL TechnicalNote 03-4.
 8. The cured polymeric coating according to claim 1, whereinthe cured second chemical composition has a contact angle for water atroom temperature greater than 90°.
 9. The cured polymeric coatingaccording to claim 1, wherein the icephobic material addition of thecured second chemical composition is composed entirely of solids. 10.The cured polymeric coating according to claim 1, wherein the icephobicmaterial addition of the cured second chemical composition comprises apolysiloxane.
 11. The cured polymeric coating according to claim 10,wherein the polysiloxane of the cured second chemical compositioncomprises an epoxy modified silicone.
 12. The cured polymeric coatingaccording to claim 1, wherein the cured second chemical composition hasa Shore D-Scale Hardness of at least
 20. 13. A non-isotropic articlecomprising: a substrate at least partly covered by a layered and/orgraded polymeric coating having a total thickness of at least 2.5microns applied to at least a part of an outer surface of the substrateand forming an exposed outer surface of the article, wherein thepolymeric coating, after curing: (i) contains an icephobic materialaddition representing up to 25% by weight of the cured polymericcoating; (ii) has a contact angle for water at room temperature greaterthan 90 degrees; and (iii) has an ice adhesion of less than 500 kPa asprepared and after 5 icing/deicing cycles when measured according toERDC/CRREL Technical Note 03-4; wherein said non-isotropic article has apull-off strength between the substrate and an exposed outer surface ofthe cured polymeric coating, according to standard ASTM 4541D, of atleast 250 psi.
 14. The non-isotropic article according to claim 13,wherein the substrate is formed of a metallic material or a polymericmaterial.
 15. The non-isotropic article according to claim 13, whereinthe cured polymeric coating has a Shore D-Scale Hardness of at least 20.16. The non-isotropic article according to claim 13, wherein theicephobic material addition, in its cured form, is composed entirely ofsolids.
 17. The non-isotropic article according to claim 13, wherein thenon-isotropic article exhibits no failure after being exposed to atleast one temperature cycle according to ASTM B553-71 servicecondition
 1. 18. The non-isotropic article according to claim 13,wherein the cured polymeric coating has a cured first chemicalcomposition and a cured second chemical composition having a differentchemical composition than the cured first chemical composition, whereinthe cured first chemical composition forms the inner surface contactingthe substrate, said inner surface is rough with a surface roughness ofR_(a)>1 micron, and wherein the cured second chemical composition formsthe exposed outer surface exposed to atmosphere, said exposed outersurface is smooth with a surface roughness R_(a)<1 micron.
 19. Anon-isotropic article comprising: (i) a substrate; and (ii) a polymericcoating compositionally graded or layered to be anisotropic in adeposition direction, having a total thickness of at least 10 microns,applied to at least part of an outer surface of the substrate, andhaving an inner surface in intimate, direct contact with the substrate,and comprising: (a) a cured first chemical composition comprising apolymeric resin at an interface between said polymeric coating and thesubstrate extending at least 2.5 microns in height from the outersurface of the substrate; (b) a cured second chemical composition havinga different chemical composition than the cured first chemicalcomposition, comprising a polymeric resin and an icephobic materialaddition forming an exposed outer surface of said non-isotropic articleextending at least 2.5 microns in depth from the exposed outer surface,wherein the cured second chemical composition: (i.b) contains theicephobic material addition composed entirely of solids; (ii.b) has aShore D-Scale Hardness of at least 20; (iii.b) has a sand erosion valueaccording to standard ASTM G76 at an impingement angle of 90 degrees ofless than 10 mm³/kg; (iv.b) is hydrophobic; and (v.b) has an iceadhesion of less than 300 kPa as prepared and of less than 500 kPa after5 icing/deicing cycles when measured according to ERDC/CRREL TechnicalNote 03-4.
 20. The non-isotropic article according to claim 19, whereinthe icephobic material addition represents up to 25% by weight of thecured second chemical composition.
 21. The non-isotropic articleaccording to claim 20, wherein said icephobic material additioncomprises a polysiloxane.
 22. The non-isotropic article according toclaim 21, wherein said polysiloxane comprises an epoxy modifiedsilicone.
 23. The non-isotropic article according to claim 19, whereinsaid polymeric coating in both the cured first chemical composition andthe cured second chemical composition, prior to curing, comprises one ormore curable epoxy resins with an EEW number of between 100 and
 750. 24.The non-isotropic article according to claim 19, wherein the pull-offstrength between the substrate and the exposed outer surface of thepolymeric coating, according to standard ASTM 4541 D, is at least 250psi.
 25. The non-isotropic article according to claim 19, wherein saidexposed outer surface is smooth with a surface roughness R_(a)≤1 micron,and wherein the cured first chemical composition forms the innersurface, said inner surface is rough with a surface roughness of R_(a)>1micron.
 26. The non-isotropic article according to claim 19, whereinsaid substrate is a metallic material or a polymeric material.